In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications networks, frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for wireless devices and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network node of the network and at the wireless devices might be required to reach a sufficient link budget.
Narrow beam transmission and reception schemes might be needed at such high frequencies to compensate the expected high propagation loss. For a given communication link, a respective beam can be applied at both the network-end (as represented by a network node or its transmission and reception point, TRP) and at the terminal-end (as represented by a terminal device), which typically is referred to as a beam pair link (BPL). One task of the beam management procedure is to discover and maintain beam pair links. A BPL (i.e. both the beam used by the network node and the beam used by the terminal device) is expected to be discovered and monitored by the network using measurements on downlink reference signals, such as channel state information reference signals (CSI-RS), used for beam management.
The reference signals for beam management can be transmitted periodically, semi-persistently or aperiodic (event triggered) and they can be either shared between multiple terminal devices or be device-specific. In order for the terminal device to find a suitable network node beam, the network node transmits the reference signal in different transmission (TX) beams on which the terminal device performs measurements, such as reference signal received power (RSRP), and reports back the N best TX beams (where N can be configured by the network). It is expected that different TX beams are transmitted in different reference signal resources (where each resource is defined in a time/frequency-grid, and that the terminal device reports back N resource indicators, such as CSI-RS resource indicators (CRIs), to inform the network node which TX beams are best. Furthermore, the transmission of the reference signal on a given TX beam can be repeated to allow the terminal device to evaluate a suitable reception (RX) beam.
There are basically three different implementations of beamforming at the TRP; beamforming, digital beamforming, or hybrid beamforming. Each implementation has its advantages and disadvantages. A digital beamforming implementation is the most flexible implementation of the three but also the costliest due to the large number of required radio chains and baseband chains. An analog beamforming implementation is the least flexible but cheaper to manufacture due to a reduced number of radio chains and baseband chains compared to the digital beamforming implementation. A hybrid beamforming implementation is a compromise between the analog and the digital beamforming implementations. As the skilled person understands, depending on cost and performance requirements of different terminal devices, different implementations will be needed. A panel might be regarded as an antenna array of single- or dual-polarized antenna elements with typically one transmit/receive unit (TXRU) per polarization. An analog distribution network with phase shifters is used to steer the beam of each panel.
One drawback with an analog beamforming implementation is that the TRP only can transmit or receive in one beam at a time (assuming one panel, and the same beam for both polarizations, which typically is the case in order to counteract dropped signal strength due to polarization mismatching).
FIG. 1 at 1), 2), and 3) schematically illustrates how a decoupled TX (at the TRP 400b of a radio transceiver device 200b implemented as a network node) and RX (at the TRP 400a of a radio transceiver device 200a implemented as a terminal device) beam sweep can be performed in order to find a BPL to be used for data transmission. In this respect, decoupled means here that the TX beam and the RX beam are determined independently of each other, as opposed to a joint sweep where every possible combination of TX beam and RX beam is tested. At 1) the network node performs a TX beam sweep by transmitting a burst of CSI-RS resources in different TX beams 140. The terminal device measures the received power for each CSI-RS resource using a wide RX beam 150 and reports which CSI-RS resource that gave the highest received power (as defined by TX beam B1 in the illustrative example). At 2) the network node transmits a burst of CSI-RS resources using the best TX beam so that the terminal device can test different narrow RX beams and find its best RX beam (as defined by RX beam B2 in the illustrative example). At 3) a BPL for data transmission can be established using the selected TX beam B1 and the selected RX beam B2.
Finding the best TX beam and the best RX beams according to the beam training procedure outlined in FIG. 1 is time consuming and causes high signaling overhead, even for a decoupled TX/RX beam sweep.
Hence, there is still a need for an improved beam training procedure.